This invention relates to a ceramic device which exhibits nonlinear voltage characteristics over a wide range of temperatures including low temperatures, and more particularly to a zinc oxide-based ceramic varistor for protecting electrical devices from voltage surges and/or transients.
In recent years, the use of large scale superconducting magnets has grown so that such devices now are finding application in particle accelerators, nuclear magnetic resonance (NMR) body-scanning systems, magnetohydrodynamic power generation, and controlled thermonuclear fusion by magnetic field containment, for example. These superconducting magnets are operated at liquid helium temperatures (4.2 K.). Such superconducting magnets are subject to voltage surges and disturbances, principally during start up operations when the magnets are up-ramped. Such disturbances take the form of point disturbances and distributed transient disturbances, the latter representing the more serious instability problem.
When such disturbances occur, the magnet might be quenched. Quenching results in the immediate vaporization of a large quantity of costly liquid helium with the attendant possibility of asphyxiation as well as resultant down time for the system. To avoid the need for quenching, elaborate and costly quench management systems have been devised which typically employ diodes in various configurations to "dump" the conductor current when the voltage rises above a certain level, i.e., when a transient or voltage surge occurs. Two diodes in parallel are often used to avoid polarity problems. Although these diodes operate at helium temperatures, they are quite large (e.g. 3 inches in diameter) in order to handle the dump current. This large size complicates heat dissipation in the diodes. Additionally, the diodes are preferably located at the fringes of the magnetic field to avoid the adverse effects of intense fields on their ferromagnetic mountings.
Zinc oxide-based compositions have been known and used widely for several years in devices designed to limit voltage surges in electrical circuits. These zinc oxide-based compositions exhibit highly nonlinear voltage/current characteristics such that as a predetermined voltage level is reached, the resistance of the composition drops drastically and permits the passage of very high current densities. The term varistor has been coined to describe the electrical behavior of such compositions and devices made therefrom.
Such zinc oxide-based varistors have been made by mixing zinc oxide with up to 20 percent by weight of other metal oxide additives such as the oxides of bismuth, antimony, cobalt, manganese, nickel, chromium, silicon, etc. and then sintering. During sintering, these metal oxide additives concentrate in the grain boundaries of the ceramic and are widely believed to provide the electrical barriers necessary to impart the nonlinear voltage/current properties. Typically, these zinc oxide-based varistors at low voltages exhibit near insulating properties (the so-called "insulating pre-breakdown region") because of the insulating barriers between grains. However, at higher voltages, the current density through the varistor may increase by up to six orders of magnitude (the so-called "breakdown region"). Finally, at still higher voltages an "upturn region" is reached where nearly linear voltage/current relationships are again found due to the highly conductive zinc oxide grains.
In contrast to other nonlinear devices such as Zener diodes and silicon carbide-based varistors, the breakdown region for zinc oxide-based devices spans a wide range of current densities. However, while these nonlinear properties exist a room temperature, such varistor properties for zinc oxide-based ceramics disappear below room temperature and certainly in the region below about 110 K.
Accordingly, the need exists in the art for a ceramic device which exhibits nonlinear voltage/current characteristics over a wide range of temperatures including low temperatures and which remains relatively unaffected by magnetic fields.